1. Field of the Invention
The invention relates to a shift control device and shift control method for a vehicle, which execute shift control over an automatic transmission and, more particularly, to a technique for shifting the automatic transmission using a shift model.
2. Description of Related Art
There is well known an automatic transmission that includes a plurality of engagement devices, which transmit rotation and torque between an input shaft that receives power from a driving force source and an output shaft that transmits power to drive wheels, and that is shifted by causing each of the engagement devices to switch between an engaged state and a released state. Generally, in such an automatic transmission, a required value (that is, a control operation amount) of an element (for example, torque, or the like) that is operated for a controlled object is adapted for each gear speed while carrying out evaluations in an actual vehicle, and the automatic transmission is shifted using the control operation amount that is determined from a control map obtained in advance for each gear speed on the basis of the adapted result. However, with an increasing number of speeds of an automatic transmission, enormous effort is required for adaptation work, and it is becoming more difficult to employ a mode of shift control based on control maps. Therefore, there is suggested shift model control that is a mode of shift control based on the equation of motion in each of rotating elements that constitute an automatic transmission. In such shift model control, a control operation amount is uniquely determined by solving the equation of motion obtained in advance on the basis of a desired variation mode (shift target value) during shifting, and a shift is carried out on the basis of the determined control operation amount. For example, Japanese Patent Application Publication No. 2000-97325 (JP 2000-97325 A) describes a technique for carrying out a shift by, in inertia phase control, setting a target value of an input shaft rotation speed of a transmission as a shift target value and calculating a required value of an engaged-side clutch torque as a control operation amount with the use of a shift model, and a technique for carrying out a shift by setting a target value of an input shaft rotation speed of a transmission and a target value of an output shaft torque of the transmission as shift target values and calculating a required value of an engaged-side clutch torque and a required value of a released-side clutch torque as control operation amounts with the use of a shift model.
Incidentally, in the technique described in JP 2000-97325 A, a shift is carried out by manipulating one controlled object for one shift target value or operating two controlled objects for two shift target values. However, in the technique described in JP 2000-97325 A, in order to cancel inertia torque during inertia phase (in other words, in order for output shaft torque during inertia phase not to substantially vary), the hydraulic pressure of the released-side engagement device is gradually decreased to release the released-side engagement device and is then temporarily gradually increased to engage the released-side engagement device again. Therefore, there is a possibility that completion of a shift delays and drivability deteriorates. On the other hand, in order to cancel the inertia torque, there is a well-known technique that is so-called engine torque reduction control for temporarily reducing engine torque during inertia phase. However, in the technique described in JP 2000-97325 A, an engine is not incorporated in the equation of motion as a controlled object. That is, in the technique described in JP 2000-97325 A, the equation of motion is solved for a current engine torque. Therefore, in the shift model control described in JP 2000-97325 A, it is not possible to cancel inertia torque through engine torque reduction control instead of a temporal increase in hydraulic pressure of the released-side engagement device. At this time, it is possible to execute engine torque reduction control separately from shift model control. However, in this case, the overall shift model control collapses, and a solution needs to be derived from the equation of motion again, so eventually there is a possibility that completion of a shift delays, a shift shock increases and drivability deteriorates. On the other hand, if an engine torque is attempted to be uniquely determined as a control operation amount in shift model control, there will be three control operation amounts for two shift target values. Therefore, it is difficult to solve the equation of motion, and it is difficult to shift the automatic transmission by using shift model control.